Guide · sizing · 9 min read
Two mistakes account for nearly every disappointing cold plunge in Indonesia: ignoring condensation, and believing the kilowatt figure on the box. Both are arithmetic, both are fixable before you buy, and both are worked through here on a real 400-litre tub.
Almost every load on a cold plunge enters through the top. A 400-litre tub of sensible proportions has about 0.88 square metres of water surface, and that single number drives most of what follows. Volume matters for the pull-down — how long the first fill takes — but it barely affects what the chiller has to shed once the tub is cold. This is why a wide shallow tub is harder to hold at 3 degrees than a narrow deep one of identical litres.
Four things arrive through that surface and two arrive elsewhere. Convection and radiation from warm air: about 14 watts per square metre for every degree between air and water. Condensation: about 190 watts per square metre of open water, every hour, all year. Conduction through the walls and base: 0.62 watts per square metre per degree through 40 millimetres of polyurethane. Then the circulation pump, which puts 85 watts of its own heat straight back into the water, and the refill water plus the bathers, about 0.012 watts per litre per degree of gap.
In a European sizing chart condensation is a rounding error, because cold water in cold dry air barely condenses anything. In Indonesia the dew point sits around 25 degrees all year. A water surface at 5 degrees is 20 degrees below the dew point, so moisture forms on it continuously, and every gram that condenses releases its latent heat into the water. On an open 400-litre tub that is roughly 167 watts — close to a quarter of the entire load, and it appears nowhere on an imported specification sheet.
An insulated lid removes most of it. Our model applies a factor of 0.58 to the whole surface term whenever the lid is on outside bathing hours, and that single component is the difference between 710 watts and 516 watts on the same tub in the same Jakarta garden. In money: Rp 283,000 a month against Rp 214,000. It is also why we include the lid in the price of every tub rather than selling it as an accessory, and why the first thing we ask a client whose tub will not hold temperature is whether anybody is putting it back on.
| Component | Closed tub | Open tub | Where it comes from |
|---|---|---|---|
| Convection and radiation from air | 171 W | 295 W | 14 W/m²K over a 24-degree gap |
| Condensation on the surface | 97 W | 167 W | 190 W/m² of open water |
| Through walls and base | 47 W | 47 W | 0.62 W/m²K through 40 mm PU |
| Circulation pump | 85 W | 85 W | pump heat dumped back into the water |
| Refill water and bathers | 115 W | 115 W | 0.012 W per litre per degree |
| Holding load | 516 W | 710 W | the number the chiller must shed, always |
Those are the real Jakarta figures for a 400-litre tub held at 5 degrees with mains water at 29 and air peaking at 34. In Bandung, where the tap runs 22-24 degrees, the same tub adds up to 432 watts, and the condensation line falls to 97 watts because the air holds less moisture to give up. In Surabaya it climbs to 529 watts. Same tub, same lid, three different machines.
A chiller is rated against 15-degree water in mild air. Push the evaporating temperature down to 5 degrees and put the condenser in 34-degree air and the same machine delivers roughly half its nameplate. Our model computes the derating explicitly rather than hiding it: start at 0.50, add 0.009 for every degree the ambient air is below 30, add 0.014 for every degree the target is above 5, and clamp the result between 0.42 and 0.78.
A chiller that exactly matches the holding load would run at 100 per cent duty forever and never recover from a session, a hot afternoon or a refill. So the first half of our rule is that effective cooling at the target temperature must be at least twice the holding load. The second half is practical: the unit must be able to take a full tub from local mains temperature down to target within eight hours, so an overnight fill is ready by morning.
| Size | Nominal | Real draw | Largest tub held at 5 °C in hot lowland Indonesia |
|---|---|---|---|
| 0.75 PK | 2.1 kW | 700 W | 180 litres |
| 1 PK | 2.8 kW | 950 W | 260 litres |
| 1.5 PK | 4.2 kW | 1,400 W | 410 litres |
| 2 PK | 5.6 kW | 1,850 W | 570 litres |
| 3 PK | 8.4 kW | 2,700 W | 1,100 litres |
That last column is the honest one, and it is the column most sellers replace with the nominal litre figure. It already includes the low-temperature capacity loss, 33-degree air, 29-degree mains water and a fitted lid. In Bandung every figure rises by about a fifth; in a highland town like Berastagi by about 45 per cent. Full specifications and prices for each unit are under ice bath chillers, and the inverter versions under inverter chillers.
Up to a point, yes, and oversizing is far less harmful than undersizing. Beyond about 2.5 times the load an on-off compressor short-cycles — it reaches temperature, stops, restarts a few minutes later — and starts are what wear a compressor out. An inverter unit avoids that because it can throttle down, which is the main argument for the inverter line at larger sizes.
For the same volume, yes. Nearly all the load enters through the water surface, so a narrow deep tub of 400 litres is cheaper to hold cold than a wide shallow one. The difference is easily one chiller size on larger formats.
On a 400-litre Jakarta tub, 194 watts of load and Rp 69,000 a month — about a third of the bill. It also keeps out leaves, dust and rain, which is why we treat it as part of the tub rather than an accessory.
First, check the obvious: lid use, condenser airflow, a blocked cartridge, and the actual voltage at the socket. Those four account for most complaints. If the machine really is too small, a smaller tub volume or a higher target temperature will bring it back into range — both are cheaper than a new compressor. We will calculate it for you even if we did not sell the unit.
On the chiller calculator, which uses this exact model with the derating table and the eight-hour rule. City water temperatures are on the locations page.
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Or size it yourself first. The calculator uses this exact model, including the derating table.
We reply with sizing, the right chiller and a full price.